INTERACTIONS BETWEEN MORPHINE AND THE OPIOID‐LIKE PEPTIDES IN THE RAT VAS DEFERENS

Abstract

1 Morphine, methadone, levorphanol, pethidine, etonitazene and related morphine-like alkaloids produced an increase in the electrically-evoked muscular contraction of the rat vas deferens. In contrast, the enkephalins and β-endorphin caused inhibition of the twitching. 2 The concentration of β-endorphin required to inhibit by 50% the muscular twitch was about 50 to 100 times less than that of the enkephalins. 3 Pretreatment of the vasa with morphine antagonized the inhibition of the neuromuscular transmission caused by either β-endorphin or enkephalin. 4 Conversely, pretreatment with β-endorphin sensitized the vasa to the increase in twitch tension caused by morphine. 5 Morphine did not alter the sensitivity to exogenously administered noradrenaline, dopamine or potassium.